1. Field of the Invention
The present invention relates to a strip line filter which is to be installed in a communication circuit or the like.
2. Description of Related Art
FIG. 4 shows a strip line filter of prior art. This filter comprises a dielectric substrate 40 and five resonators which are in serial electrical connection with each other, and functions as a bandpass filter. A grounding electrode 44 is formed entirely on one major surface (back surface) 40b of the dielectric substrate 40. On the other major surface (front surface) 40a of the dielectric substrate 40, a pair of guard electrodes 46 and 47, a plurality of resonance electrodes 51, 52, 53, 54 and 55 and input/output electrodes 58 and 59 are formed. A capacitance generated between the resonance electrode 51 and the grounding electrode 44 and the inductance of the resonance electrode 51 form an LC resonance circuit. In other words, the resonance electrode 51, the dielectric substrate 40 and the grounding electrode 44 form a resonator. Likewise, the resonance electrodes 52 through 55, the dielectric substrate 40 and the grounding electrode 44 form four resonators.
The guard electrodes 46 and 47 are formed on sides and edges of the dielectric substrate 40 and electrically connected with the grounding electrode 44. The resonance electrodes 51 through 55 are extended from the guard electrodes 46 and 47 alternately, and each of the resonance electrodes 51 through 55 is a strip which has a fixed width. The resonance electrodes 51 through 55 are arranged at a constant pitch, and the electrodes 51 and 55 are connected with the input/output electrodes 58 and 59 respectively.
The intervals S5 among the resonance electrodes 51 through 55 and the length L5 of the resonance electrodes 51 through 55 are determined depending on the center frequency and the bandpass width of the filter. In order to obtain a good bandpass characteristic, the intervals S5 among the resonance electrodes must be sufficiently large, which results in enlargement of the size of the filter. As measures to prevent this, it has been suggested that a material with a small dielectric constant is used for the dielectric substrate 40. By use of such a material, the intervals S5 do not have to be large, whereas the length L5 of the resonance electrodes must be larger such that the capacitances generated between the resonance electrodes 51 through 55 and the grounding electrode 44 will not be smaller. Thus, this does not practically contribute to downsizing of the filter.